Summary Ministic Flight 303, a Beech 1900D, serial number UE-233, was departing runway 12 at the Island Lake airport, en route to Winnipeg, Manitoba. The aircraft was carrying a crew of two, and 17 passengers. Take-off acceleration was described as normal. As the aircraft was rotated for take-off, the stall warning horn activated. The take-off was rejected, reverse thrust was selected, and both pilots applied the aircraft's brakes. The aircraft did not stop within the confines of the runway or the stopway area and departed off the end of the prepared surface. The aircraft came to rest straddling a ploughed bank of snow and sustained substantial damage. The engines were shut down, and the passengers and the crew evacuated the aircraft. One minor injury was reported. The occurrence took place during daylight hours, at 1410 central standard time (CST)(1). Ce rapport est galement disponible en franais. Other Factual Information Both the captain and the first officer were certified and qualified for the flight in accordance with existing regulation. The captain had a total flight time of 5 200 hours, 704 hours of which were on type. The first officer had a total time of 1 250 hours, 200 of which were on type. The captain had received most of his training on the Beech 1900D type at an outside facility which made extensive use of a flight simulator. The first officer had received his type training in-house, most of which was undertaken by a contract instructor. Neither pilot reported receiving instruction on faults related to the stall warning system which could result in a false stall warning. The flight was a continuation of a series of flights originating at Winnipeg as Ministic Flight 302, with scheduled station stops at St. Theresa Point and Island Lake. The aircraft entered snow conditions at the start of the descent into St. Theresa Point, and visibility was observed to be about one-half mile in snow during the approach. The station stop at St. Theresa Point lasted about 25 minutes, during which time several passengers and some freight items were deplaned, and other passengers embarked. Snow continued to fall during the station stop, and the crew used brooms to remove snow from the aircraft's wings before departure. Snow was falling throughout the flight to Island Lake and the station stop of about 30 minutes at Island Lake. Observers noted some loose snow on the wings as the aircraft taxied from the ramp to the runway. Both crew members reported that this snow was not adhering to the wings, and that it blew off of the wings with the movement of the aircraft after it taxied from the ramp at Island Lake. The first officer was at the controls during the take-off from Island Lake, and the captain performed the non-flying duties. These duties included monitoring the engine instruments and calling out the reference speeds during the take-off roll. The crew used a cockpit quick-reference chart, with speeds as follows: V1 (take-off decision speed), 103 knots, VR (rotation speed), 106 knots, and, V2 (single engine climb speed), 111 knots. A take-off is not normally rejected after V1 unless the aircraft's ability to fly is in doubt. The chart was produced by another operator and the figures in it were derived from the performance section of the Transport Canada approved aircraft flight manual (AFM). The figures in the quick-reference chart vary for every 1 000 pounds of gross weight and were calculated for an outside air temperature (OAT) of 25 degrees Celsius. For the take-off from Island Lake, the crew selected take-off speeds from the quick reference chart for a take-off weight from 16 001 pounds to 16 950 pounds. The take-off speeds listed in the AFM vary for every five degrees Celsius change in the OAT and every 1 000 pounds of gross weight. For paved, dry runway conditions, the AFM designated take-off speeds for an aircraft at a take-off weight of 16 000 pounds and an outside air temperature of minus five degrees Celsius were: V1, 100 knots, VR, 102 knots, and V2, 108 knots. The aircraft's balanced field length under these conditions is listed as 3 328 feet. The AFM contains a gravel supplement with performance information for aircraft using firm, dry, gravel surfaces. The ravel supplement portions of the AFM list the following take-off speeds for the occurrence aircraft's weight and an outside air temperature of zero degree celsius: V1, 101 knots, VR, 101 knots, and V2, 108 knots. The balanced field length for these conditions is listed as 3 484 feet. The aircraft was equipped with both a flight data recorder (FDR) and a cockpit voice recorder (CVR). Both units were removed from the aircraft and their data were analysed. The FDR indicated that, during the start of the take-off roll, both engines were producing rated torque and both propellers were turning at maximum rated rpm. The FDR indicated that the aircraft was rotated at an indicated airspeed of about 106 knots. On hearing the stall warning horn, the first officer suspected that the aircraft may have been over-rotated and lowered the nose. The stall warning stopped, but recurred when the nose was raised again. The first officer then believed that a malfunction had occurred which would compromise the aircraft's flight capability. He called for a rejected take-off, and the captain concurred. The captain moved the engine power levers to idle and applied reverse thrust. FDR data indicate that engine power reduced to idle about four seconds after the aircraft was rotated. The aircraft's indicated airspeed reached a maximum of 126 knots and then declined sharply. The indicated airspeed was about 40 knots as the aircraft travelled past the end of the runway, and 20 knots as it departed the prepared surface of the stopway. FDR data indicate that the elapsed time to accelerate from 101 knots to 106 knots during the take-off roll was about one second, and a similar time was required for the deceleration through those speeds. The FDR data do not indicate with certainty whether or how far the aircraft lifted from the ground, and witness reports were inconclusive. When the aircraft was examined after the occurrence, some ice was observed on the engine cowlings and on both wing sections, inboard of the engines; however, no ice or snow was found on the tail surfaces or the wings outboard of the engines. The design of the aircraft incorporates an engine bleed air system, heat exchanger and air cycle machine, much of which is located in the wing roots. When these systems are in operation, they generate heat, which has the effect of warming the skin of the inboard wing sections. The runway at Island Lake is 4 000 feet long, composed of crushed stone. In addition to the runway length, there is a stopway area of about 300 feet on each end of the runway. The stopway is cleared of snow in winter, and is used by flight crews to turn their aircraft around before take-off and after landing. The area beyond the stopway area of runway 12 is an unprepared surface sloping down toward the lake. On the day of the occurrence, this area contained several banks of hard snow of various heights, and the runway and stopway surfaces were covered with graded, hard-packed snow. About one to two inches of loose snow was observed on the surface of the runway at the time the aircraft taxied for take-off. The runway surface was described as slippery at the time of the occurrence. Loose snow increases tire rolling resistance, delays acceleration, and results in longer take-off runs. Snow-covered or slippery runways provide decreased traction, which results in longer aircraft stopping distances, as compared to bare runways. The calculated weight of the aircraft at take-off from Island Lake was 16 015 pounds, 935 pounds less than the aircraft's maximum gross take-off weight of 16 950 pounds. Its centre of gravity was within approved limits. The aircraft's maintenance records indicate that it was equipped and maintained in accordance with existing regulations. Transport Canada approved the Beech 1900D aircraft type for operation in Canada under section 704 of the Canadian Air Regulations. Section 704 provides that no person shall conduct a take-off in an aircraft if the weight of the aircraft exceeds the maximum take-off weight specified in the AFM for the pressure altitude and the ambient temperature at the aerodrome where the take-off is to be made. In the determination of the maximum take-off weight, the required accelerate-stop distance shall not exceed the accelerate-stop distance available, and the required take-off distance shall not exceed the take-off distance available. For the purposes of determining the accelerate-stop distance and take-off distance, the following factors shall be taken into account: the pressure altitude at the aerodrome, the ambient temperature, the runway slope in the direction of take-off, and headwind and tailwind components. The manufacturer was, as a condition of the aircraft type approval, required to determine and supply certain aircraft performance data including the balanced field lengths at various take-off weights, temperatures, and altitudes. The stopping performance of the aircraft for the accelerate-stop distance is calculated with engine power at idle, without the use of reverse thrust. The James Brake Index (JBI) published in the Canada Flight Supplement contains a table which may be used to adjust calculated landing distances to compensate for slippery braking conditions. JBI correction factors for compacted snow or snow-covered runways range from 80% to 250% higher than hard dry surfaces. Transport Canada did not require the manufacturer to provide data on the effects of soft or wet runways, slippery runways, or runways containing loose snow on the aircraft's accelerate-stop distances or take-off distances. Slippery and snow-covered runways are commonly encountered by flight crews operating in Canada during cold weather. The operator did not have performance charts for use for such conditions, nor were such charts available from the manufacturer. The Island Lake weather observation at 1400 was as follows: winds 080 degrees true at eight knots, visibility one-half statute mile in snow, an overcast cloud ceiling at 500 feet above ground level, and a temperature of minus four degrees Celsius. The observations at 1450 and 1443 noted visibilities of one mile and two miles in light snow, respectively. As the crew was taxiing the aircraft to the runway for takeoff, they requested updated weather information from the Winnipeg Flight Service Station. They were advised that the current ceiling was 700 feet above ground level, and the visibility was one statute mile. Snow continued to fall throughout the afternoon on the day of the occurrence. The aircraft's stall warning system consists of a lift transducer vane and a backing plate located on the left wing leading edge, a sensor unit, and several resistors. The vane is able to move up or down within a range of motion afforded by a gap in the backing plate in which it is mounted. Aerodynamic pressure on the lift transducer vane varies with the wing's angle of attack. When an angle of attack approaches that of an imminent stall, the vane changes position, and the sensor unit produces a signal which activates the stall warning horn in the cockpit. Rigging tolerances allow the vane to be in the up, wing stalled, position or the down, wing unstalled position on the ground. In unstalled flight, dynamic air pressure holds the vane in the down, or wing unstalled position. The system is disabled on the ground by the operation of the landing gear safety switch, located on the left main landing gear. The system has a preflight test capability through the use of a switch placarded STALL WARNING TEST on the copilot's left hand subpanel. The stall warning test is incorporated into the originating check, which is performed before the first flight of the day, but not at station stops. The stall warning system was tested before the initial flight on the day of the occurrence and found to be serviceable. This switch, when held in the TEST position, bypasses the landing gear safety switch, and, if the system is functional, activates the stall warning horn. The test system does not detect a system malfunction which would generate a false stall warning in flight. The Beech 1900 series stall warning system differs from some other stall warning systems in that the lift transducer vane in the Beech 1900 may be in the stalled or unstalled position while the aircraft is at rest on the ground. In the Beech 200 system, for example, the vane is normally in the unstalled position on the ground. The stall warning heat is switched on and checked along with other ice protection items as part of the BEFORE TAKE-OFF (FINAL ITEMS) check in the Transport Canada approved check list, which was in effect at the time of the occurrence. The stall warning heat is switched off as part of the AFTER LANDING check. The crew reported that the stall warning heat was switched on at Island Lake as they taxied the aircraft from the ramp to the runway for take-off. The aircraft taxied away from the ramp about two minutes before the start of the take-off roll. The stall warning system is equipped with anti-icing capability on both the mounting plate and the vane. The heat is controlled by a switch in the ICE PROTECTION group located on the pilot's right subpanel placarded STALL WARN - OFF. Electrical voltage is supplied to the stall warning heat system at 28 volts in the air, and is reduced to 10 volts for ground operation by the operation of the left landing gear safety switch. The manufacturer's information does not quantify the temperatures attained by the system during ground or air operation; such temperatures would depend on ambient temperature, atmospheric moisture, and relative wind. The AFM states that the level of stall warning heat is minimal for ground operation. A STALL HEAT annunciator in the Caution/Advisory panel illuminates if there is insufficient current to heat the vane and the faceplate heaters. No STALL HEAT indication was observed by the crew on the day of the occurrence. Several hours after the occurrence, the stall warning transducer vane was checked by the crew, and found to be stuck. The ambient air temperature did not rise above freezing from the time of the occurrence until the aircraft was examined the following day by TSB investigators. During that examination, the vane was found to be frozen in the wing stalled position. Power was supplied to the stall warning heat system with the landing gear safety switch in the ground position. The lift transducer vane and its backing plate gradually became warm to the touch, but remained frozen for several minutes after heat was applied. When the landing gear safety switch was moved to the flight position, the temperature of the vane and backing plate increased rapidly, and the vane became free to move. After the stall warning vane was freed, it remained in the wing stalled position with the aircraft at rest. There have been a number of similar occurrences both in Canada and United States where the stall warning horn activated during the take-off sequence, and in some cases after the aircraft had been de-iced.